1. Field of Technology
The present disclosure relates to fatigue resistant titanium-base alloys and articles of manufacture including the alloys.
2. Description of the Background of the Technology
There are approximately 30 different metallic biomaterials that have been used or that are being considered for use to manufacture implantable medical and surgical devices. These distinctly different metallic biomaterials are differentiated by their chemical compositions and by their mechanical and metallurgical properties as defined by international ASTM Standards, ISO Standards, and UNS designations. The 30 metallic biomaterials can be categorized into four groups: stainless steels (iron-base alloys); cobalt-base alloys; titanium grades; and specialty grades.
Before the advent of implantable orthopedic and cardiovascular devices, metallic materials had first been developed for use in applications in other industries in which corrosion resistance and heat-resistance was needed. Certain improved corrosion resistant stainless steels developed for the chemical industry and certain cobalt-base alloys developed for the aerospace industry are examples of cross-industry application of metallurgical technology to the earliest medical implants for total joint arthroplasty. Dr. John Charnley's pioneering work with stainless steel hip stems in the 1960s was followed by experimentation with titanium and zirconium materials. Those early materials that were proven successful in medical device applications were defined in the first ASTM F04 “metallurgical materials” standards (ASTM F04.12), and those standards were derived from published chemical industry and aerospace industry standards. These early “medical” materials were later designated as “grandfathered” material grades in ASTM F 763 (see Table 1) and are commonly considered, each on its own merit, as a reference metallic biomaterial against which any new implantable metallic biomaterial is compared.
TABLE 1Common NameASTM and ISO StandardsUNS Number(s)Unalloyed TitaniumASTM F 67, ISO 5832-2R50250, 400, 550, 700Grades CP-1, 2, 3, 4Co—28Cr—6Mo CastingsASTM F 75, ISO 5832-4R30075and Casting AlloyCo—20Cr—15W—10Ni—1.5MnASTM F 90, ISO 5832-5R30605(“L-605”) AlloyTi—6Al—4V ELI AlloyASTM F 136, ISO 5832-3R56401Fe—18Cr—14Ni—2.5MoASTM F 138, ISO 5832-1S31673(“316 LS”) Alloy35Co—35Ni—20Cr—10MoASTM F 562, ISO 5832-6R30035(“MP-35N”) Alloy
In the last 15 years, there have been important additions of new alloys to each of the four basic metals groups as improved and new biomedical devices and applications have been developed. Three newer wrought stainless steel alloys, listed below in Table 2, are now being used in approved medical and surgical devices. Table 2 also lists certain trade names that have been used with the alloys. Criteria for these stainless steel grades included improved corrosion fatigue properties, reduced nickel content, and ductility similar to or improved over existing biomedical stainless steel grades. All three of these alloys were the subject of patents, which have since expired.
TABLE 2Fe—21Cr—12.5Ni—5Mn—2.5Mo(“XM-19”, ASTM F 1314, UNS S20910)Fe—22Cr—10Ni—3.5Mn—2.5Mo(“REX 734”, ASTM F 1586, UNS S31695)Fe—23Mn—21Cr—1Mo—1N(“108”, ASTM F 2229, UNS S29108)
Certain important alloy development projects directed to cobalt-base alloy systems have resulted in novel chemistry and processing advances and improved cobalt-base alloys. One such development project applied an older alloy that had been used as a spring wire in the Swiss watch industry to biomedical applications, followed by like application of two fairly similar grades. See ASTM F 563, “Standard Specification for Wrought Cobalt-20 Nickel-20 Chromium-3.5 Molybdenum-3.5 Tungsten-5 Iron Alloy for Surgical Implant Applications (UNS R30563)”; and ASTM F 1058, “Standard Specification for Wrought 40 Cobalt-20 Chromium-16 Iron-15 Nickel-7 Molybdenum Alloy Wire and Strip for Surgical Implant Applications”, Annual Book of ASTM Standards. Subsequently, three variations on the cast Co-28Cr-6Mo alloy were developed, and each is covered by a wrought CoCrMo alloy standard, ASTM F 1537. The ASTM F 1537 standard was an outgrowth of the ASTM F 799 standard, which was originally for a forging and machining alloy having a chemistry almost identical to the ASTM F 75 standard, which is for the casting alloy and castings. Alloy #3 in the ASTM F 1537 standard represents a CoCrMo grade with small additions of aluminum and lanthanum oxides. Patents for this gas atomized, dispersion strengthened (“GADS”) alloy discuss methods of manufacture and improved properties of the alloy in the forged and sintered conditions. See U.S. Pat. Nos. 4,714,468 and 4,687,290. More recently, several patents were issued for a single-phase ASTM F 1537 Alloy #1 with improved high cycle fatigue properties. See U.S. Pat. Nos. 6,187,045, 6,539,607, and 6,773,520. Similarly, a higher fatigue version of the 35Co-35Ni-20Cr-10Mo (ASTM F 562) alloy has been introduced for wrought and drawn product forms. See Bradley, et al. “Optimization of Melt Chemistry and Properties of 35 Cobalt-35 Nickel-20 Chromium-10 Molybdenum Alloy (ASTM F 562) Medical Grade Wire,” ASM International M&PMD Conference, Anaheim, Calif., September 2003. Various alloys discussed above and related common trade names are listed below in Table 3.
TABLE 3Co—20Ni—20Cr—5Fe—3.5Mo—3.5W—2Ti(“Syncoben”, ASTM F 563, UNS R30563)Co—20Cr—15Ni—15Fe—7Mo—2Mn(“Elgiloy”, ASTM F 1058, UNS R30003)Co—19Cr—17Ni—14Fe—7Mo—1.5Mn(“Phynox”, ASTM F 1058, UNS R30008)Co—28Cr—6Mo(“GADS”, ASTM F 1537, Alloy #3, UNS R31539)Co—28Cr—6Mo(“No-Carb”, ASTM F 1537, Alloy #1, UNS R31537)35Co—35Ni—20Cr—10Mo(“35N LT”, ASTM F 562)
Significant change has occurred in the use of titanium and titanium alloys and the number of new titanium materials and product forms the medical device designer has from which to select. Since the early 1990s, several new ASTM standards for titanium-base alloy biomaterials have been developed by the “Metallurgical Materials” Subcommittee, ASTM F-04.12. These consensus standards, listed below in Table 4, have been balloted and approved by the “Medical and Surgical Materials and Devices” Main Committee, F-04. One such standard, ASTM F 1295, is directed to an α+β titanium alloy, which originally was invented in Switzerland and has intrinsic properties similar to the two “Ti-6-4” alloys, but uses niobium instead of vanadium as a P stabilizing alloying element. A second new standard, ASTM F 1472, is directed to biomaterial applications of the most widely produced aerospace titanium grade, Ti-6Al-4V alloy (UNS R56400).
ASTM F 1713 and F 1813, working through subcommittees simultaneously, were for two entirely new metastable β titanium alloys with properties designed by medical device manufacturing companies specifically for structural orthopedic implant applications. The ASTM F 2066 standard was developed for the metastable β titanium alloy, titanium-15 molybdenum (Ti-15Mo). ASTM F 2146 covers low-alloy α+β Ti-3Al-2.5V tubing used for medical devices, which is based on a product used for aerospace hydraulic tubing for more than 40 years.
TABLE 4Common NameASTM/ISOMicrostructureUNS NumberTi—5Al—2.5Fe Alloy (“Tikrutan”)ISO 5832-10α + βunassignedTi—6Al—7Nb Alloy (“TAN”)ASTM F 1295,α + βR56700ISO 5832-11Ti—6Al—4V AlloyASTM F 1472,α + βR56400ISO 5832-3Ti—13Nb—13Zr AlloyASTM F 1713metastable βR58130Ti—12Mo—6Zr—2Fe Alloy (“TMZF”)ASTM F 1813metastable βR58120Ti—15Mo AlloyASTM F 2066metastable βR58150Ti—3Al—2.5V Alloy (tubing only)ASTM F 2146α + βR56320Ti—35Nb—7Zr—5Ta Alloy “TiOsteum”Sub. F-04.12.23metastable βR58350
Another metastable β titanium alloy, Ti-35Nb-7Zr-5Ta, was developed specifically for structural orthopedic implants, such as total hip and total knee systems, with the objectives of overcoming some of the technical limitations of the three established α+β titanium alloys. With titanium, niobium, zirconium, and tantalum as alloying elements, the superior corrosion resistance and osseointegratabilty of this alloy have been demonstrated. See Hawkins, et al., “Osseointegration of a New Beta Titanium Alloy as Compared to Standard Orthopaedic Implant Materials,” No. 1083, Sixth World Biomaterials Congress, Society for Biomaterials, May 2000; Shortkroff, et al., “In Vitro Biocompatibility of TiOsteum,” No. 341, Society for Biomaterials, Brigham and Women's Hospital and Harvard Medical School, April 2002.
Despite the wide variety of titanium-base and other biomaterials currently available and being developed, there remains a need for further improved materials for medical and surgical applications. For example, improvements in cyclic fatigue strength and certain other mechanical properties of biocompatible titanium-base materials would be particularly helpful in fabricating improved medical implants subjected to high and/or cyclic stresses. Any such improved alloys, however, must still provide sufficient ductility appropriate for the intended application for the medical or surgical device. For example, orthopaedic surgeons in trauma cases may need to shape bone plate implants made of these improved alloys to suit the needs of the patients (for example, intraoperative contouring of metal plates or rods). Improved alloys also must exhibit a suitable modulus of elasticity so as to sufficiently replicate the performance of the human bones or tissues they replace or repair.
More generally, there remains a need for titanium-base alloys having improved properties and/or reduced production cost and which may be used in one or more of a variety of applications including, for example, biomedical, aerospace, automotive, nuclear, power generation, costume jewelry, and chemical processing applications.